1. Field of the Invention
The present invention relates to an exposure control apparatus and method for controlling the exposure value onto a photosensitive substrate and, more particularly, to exposure control of an exposure apparatus of a slit-scanning exposure type for exposing a pattern on a mask onto a photosensitive substrate by illuminating a rectangular or arcuated illumination region with light pulses from a pulse light source, and synchronously scanning the mask and the photosensitive substrate with respect to the illumination region.
2. Related Background Art
Conventionally, in the manufacture of a semiconductor element, a liquid-crystal display element, a thin-film magnetic head, or the like using a photolithography technique, a projection exposure apparatus for exposing a pattern on a photomask or a reticle (to be referred to as a "reticle" hereinafter) onto a photosensitive substrate such as a wafer or glass plate coated with, e.g., a photoresist via a projection optical system is used. Recently, the size of a single chip pattern (one shot area radiated onto a wafer) on a semiconductor element tends to become large, and the projection exposure apparatus is required to expose a pattern having a larger area on a reticle onto a photosensitive substrate (large area requirement). Also, it is required to increase the resolution of the projection optical system in correspondence with a decrease in line width of a pattern of, e.g., a semiconductor element.
However, it is not easy to increase the resolution of the projection optical system and to simultaneously increase the size of an exposure field of the projection optical system. In particular, when a catadioptric system is used as the projection optical system, an aplanatic exposure field often has an arcuated shape.
In order to meet the above-mentioned large area requirement of a pattern to be transferred and limitation on the exposure field of the projection optical system, a projection exposure apparatus of a slit-scanning exposure type has been developed. In this apparatus, by synchronously scanning a reticle and a photosensitive substrate with respect to, e.g., a rectangular, arcuated, or hexagonal illumination region (to be referred to as a "slit-shaped illumination region"), a pattern, having an area wider than the slit-shaped illumination region, on the reticle is exposed onto the photosensitive substrate. In general, in a projection exposure apparatus, since an appropriate exposure value for a photosensitive material on a photosensitive substrate is determined, the projection exposure apparatus of the slit-scanning exposure type comprises an exposure control apparatus for controlling the exposure value with respect to the photosensitive substrate to coincide with an appropriate exposure value within a predetermined allowable range.
As one technique for increasing the resolution of a pattern to be exposed onto a photosensitive substrate, a technique for decreasing the wavelength of exposure light is known. In association with this technique, of existing light sources, those which emit light having a short wavelength are pulse-oscillation type laser light sources (pulse-oscillation light sources) such as an excimer laser light source, a metal vapor laser light source, and the like. However, unlike a continuous emission type light source such as a mercury lamp, energy of light pulses emitted from a pulse-oscillation light source varies within a predetermined range in units of pulse emissions.
Therefore, in the conventional exposure control apparatus, when the average energy of light pulses emitted from the pulse-oscillation light source is represented by pa, and the range of a variation in pulse energy of the light pulses is represented by .DELTA.p, it is assumed that a parameter .DELTA.p/pa representing the variation in pulse energy has a normal distribution (is random). When the number of light pulses radiated onto a certain region (to be referred to as a "pulse count integrating region" hereinafter) on a photosensitive substrate which is scanned relative to an exposure region conjugate with a slit-shaped illumination region illuminated with light pulses is represented by n, by utilizing the fact that a variation in integrated exposure value after the end of exposure is given by (.DELTA.p/pa)/n.sup.1/2, the integrated exposure value is controlled to reach an appropriate exposure value within a predetermined allowable range under the assumption that the variation (.DELTA.p/pa) in pulse energy does not exceed a predetermined value. For example, when .DELTA.p/pa three times a standard deviation .sigma. is assumed to be 10%, in order to set a desired reproduction precision A of an integrated exposure value three times the standard deviation .sigma. to be 1%, n is 100 or more. Therefore, it suffices if the reticle and the photosensitive substrate are synchronously scanned relative to a slit-shaped illumination region, so that the number of light pulses radiated onto each pulse count integrating region on the photosensitive substrate becomes 100 or more.
However, since conventional exposure value control is open control, when the oscillation state of the pulse-oscillation light source fluctuates for some reason, and the variation (.DELTA.p/pa) in pulse energy temporarily exceeds 10%, the desired reproduction precision A of the integrated exposure value can no longer be obtained.
In order to solve this problem, in a projection exposure apparatus such as a stepper for exposing a pattern on a reticle onto a photosensitive substrate while the reticle and the photosensitive substrate stand still, as disclosed in commonly assigned Japanese Laid-Open Patent Application No. 63-316430 and U.S. Pat. No. 4,970,546, a modified exposure method for performing exposure by reducing some last light pulses, and a cutoff method for ending exposure when the integrated exposure value reaches an appropriate exposure value within a target precision range are known. In the cutoff method, the number of light pulses radiated onto the photosensitive substrate is not constant. Furthermore, as filed in commonly assigned U.S. patent application Ser. No. 623,176 (May 12, 1990), a technique for controlling an exposure value by finely adjusting pulse energy in units of pulses is also known.
However, due to the unique feature of the projection exposure apparatus of the slit-scanning exposure type, that is, since light pulses radiated on a plurality of pulse count integrating regions on the photosensitive substrate have different integrated energy levels, the above-mentioned exposure value control method proposed for a non-scanning type exposure apparatus cannot be directly applied.